Thanks for your answer. I also found out that no error is thrown if I try to connect. I'm also able to transfer files and so on. But I do not think that there is actually data compression ... I did not noticed a significant change in transfer rate.

I have checked it against openssh client + openssh server transferring files with a gzip compression ratio of 90% where I noticed a performance boost.

I assume that the negotiation during connect might fail... and compression is turned off. That’s why I asked if the compression is compatible (and working).

You are correct. There was indeed a bug in earlier versions (5.0-6.1.7) of Tectia Server where compression was not actually used even when requested by the client. This was fixed in version 6.1.8 and above of Tectia Server. Which version are you using?